(A) Trypansoma cruzi clones, exhibit marked differences in isoenzyme profiles and growth kinetics which are stable and diagnostic of the source of the parasite. Organism from all clones isolated thus far are capable of infection vertebrate cells. Clones derived from chronic human infections have extermely long doubling times (about 8 days) resulting in an intracellular scale of about 3 months duration. Clone-derived trypomastigotes are capable of infecting insects. The rate of transformation in the insect varies between clones. (B) The initiation of RNA synthesis in T. cruzi does not begin until completion of trypomastigote-amastigote reorganization. Therefore, the mRNA required for this transformation is present in the trypomastigote. After reorganizing, amastigotes synthesize the RNA required for intracellular growth and reproduction during the prereplicative lag period. (C) Clones of T. cruzi have been established which consistently undergo nearly 100% epimastigote-metacylcic trypomastigote transformation in liquid culture medium.